Eccentric rotary plug valves are well known and represent a highly developed area of technology. In such valves a plug or closure member is mounted for rotation in a generally cylindrical body with the axis of plug rotation slightly offset from the axis of the valve body, or more specifically, offset from the axis of that part of the body against which the plug closes--hence the eccentric nature of the valve.
Plug valves have inlets and outlets which usually are aligned and typically are sized to connect with pipes ranging from 3 to 48 inches in diameter. Normally the plug or closure member is rotated from an open position, at one side of the cylindrical valve body, toward the interior face of the valve inlet where it becomes seated to fully close the valve. So as to minimize sliding contact of the respective plug and mating seat surface and provide seating adjustment, it has been known to make that portion of the interior cylindrical surface surrounding the valve inlet slightly eccentric to the valve body axis (such a by generating that portion of the interior surface from an axis offset from the axis of plug rotation), and to also make the corresponding cylindrical closure face of the plug slightly eccentric to the axis of rotation which, in this construction may coincide with the valve body axis. The seating end of the valve is hereinafter referred to as the inlet end; however, it may actually be at either the inlet or outlet end depending on the orientation of the valve in the pipeline.
Most often either the plug or mating body seat surfaces have fused or welded seats of relatively harder and resistant metal which are machined in place to a smooth finish. Such seats have several disadvantages including high manufacturing cost, corrosion susceptibility and difficulty, if not impossibility, to replace or renew upon excessive wear. Also when the valve seat is at the inlet end, hydraulic pressure against the plug of the closed valve tends to separate the plug from the body seat and if any play develops in the plug, there will be leakage around the seat. U.S. Pat. No. 2,574,428 appears to disclose a replaceable composite valve seat which positions a resilient ring for contact by a cast plug closure which is not described in detail. U.S. Pat. No. 4,697,786 discloses a recent attempt to devise at least one form of a replaceable plug valve seat. However, that form of valve requires a composite machined metal plug closure, and the replaceable seat in the valve body inlet is believed likely to be displaced by the pressure of liquid as the valve is partially opened.